feature(email): send email with react component will now render text if text is undefined

[sommeeeer]

Closes #451 and #369

When you pass a React component to either resend.emails.send() or resend.batch.send() we want it to automatically send the text version if text is undefined. This PR solves that.

const email = await resend.emails.send({
    from: 'test <[email protected]>',
    to: '[email protected]',
    subject: 'hey',
    react: <Email url="https://example.com" />,
    // text: will be automatically rendered here
  });